With the recent increase in the consumption of nitrogen fertilizers, the contamination of groundwater and the like with nitrogen compounds such as nitric acid has also increased. For instance, in several cases in European and American countries, young children who had drunk water containing nitrate died of anemic blue baby syndrome. The gastric juice in the stomachs of these young children was not acidic; the nitric acid converted into nitrous acid due to microorganisms in the stomach, and the nitrite bonded to hemoglobin in the blood. Thus, the young children died because of insufficient oxygen supply.
The World Health Organization (WHO) has determined the acceptable standard concentration of nitrate nitrogen to be 10 ppm. However, groundwater in many European and American countries actually far exceeds this standard. For example, approximately 8% of all wells in Denmark have detectable nitrate nitrogen concentrations as high as 50 ppm. Further, nitrate nitrogen concentrations as high as 150 ppm have been detected in groundwater from the United States. Furthermore, because groundwater in some cultivated fields in Japan has been found to contain nitrate nitrogen concentrations in amounts exceeding the standard of 10 ppm, the Japanese Ministry of Public Welfare and Environmental Agency have heightened their monitoring of nitrate nitrogen concentrations.
Further, treatment of wastewater such as sewage has also led to increased eutrophication of lakes and rivers. Despite improvements and amplification of sewer facilities, the fundamental problems have not been solved. An activated sludge treatment has been employed; this treatment can decrease the biochemical oxygen demand (BOD) attributable to microorganisms but cannot remove nutrients such as nitrogen compounds, phosphorus, or the like, which were derived from organic compounds contained in the wastewater. When these nutrients are released into lakes and rivers, plant planktons multiply, eutrophication continues, and the lakes and rivers are further contaminated.
Various methods are applied during drainage of groundwater, wastewater and the like to remove the ammonia nitrogen, nitrate nitrogen and nitrite nitrogen converted from harmful organic nitrogen. In the biochemical denitrification method, treatment occurs in a nitrification treatment tank or a denitrification treatment tank using nitrifying bacteria or denitrifying bacteria present in an activated sludge. The nitrification treatment tank can convert organic nitrogen into ammonia nitrogen, and can also convert the ammonia nitrogen into nitrate nitrogen or nitrite nitrogen using nitrifying bacteria having a slow growth rate. The nitrate nitrogen and nitrite nitrogen which were converted from the ammonia nitrogen in the nitrification treatment tank can be further converted into nitrogen in a denitrification treatment tank using denitrifying bacteria supplied with a hydrogen donor; the nitrogen is thus released into the atmosphere. Thus, nitrogen compounds are removed from groundwater and wastewater. In previous attempts to increase treatment capacity, the introduction of a polyurethane foam or a carrier for immobilizing microorganisms prepared from jelly-like polyethylene glycol derivatives as disclosed in JP-A-5-023684 into the nitrification treatment tank has been proposed. ("JP-A" refers to an unexamined published Japanese patent application.) The use of a cellulose foam as a carrier for immobilizing microorganisms to produce useful material such as citric acid has also been proposed.
However, conventional methods have the following problems. The method of using a carrier prepared from polyethylene glycol derivatives, as disclosed in JP-A-5-023684, requires prior immobilization of microorganisms onto the carrier. This requires a plant investment in an apparatus for immobilizing microorganisms. The carrier prepared from polyethylene glycol derivatives can immobilize nitrifying bacteria thereon in the nitrification treatment tank under aerobic conditions, but cannot immobilize denitrifying bacteria thereon in the denitrification treatment tank under anaerobic conditions. Thus, this conventional method cannot be widely used.
When a polyurethane foam is used in the denitrification treatment tank, it can overcome the disadvantage of the polyethylene glycol derivatives, but the polyurethane foam does not have a sufficient affinity for the above-mentioned microorganisms. Therefore, the maximum nitrification rate of the polyurethane foam carrier used in the nitrification treatment tank is as low as 130 (mg-N/l-carrier/hr); further increases in the treatment capacity are difficult to imagine. In addition, since the polyurethane foam, after use, is synthetic resin, the disposal of the polyurethane foam is problematic.
Further, the cellulose foam has a markedly short life in an environment where cellulose degradation activity is high, and it does not withstand use for several months. Thus, the cellulose foam is not suitable for many purposes.